Solder alloy having a quasi Sn—Pb eutectic structure is widely used because of the low melting point and reliability thereof. However, the demand for lead-free solder is rising up because of the standpoint of environment protection. Lead-free solder having Sn—Cu structure currently in widespread use, in particular, Sn—Cu—Ni structure offers a higher fluidity than other lead-free solder. The Sn—Cu—Ni solder is favorable because it is free from soldering failures including smoothing failure of the soldering surface, solder bridge, through hole joint, dry joint, and so on, which could be problematic during bulk production.
A printed circuit board with Cu film coated of electronic apparatuses, and electronic components having Cu lead wires or Cu tapes are immersed into a solder dipping bath for soldering. Cu can dissolve into the solder dipping bath, thereby gradually raising the Cu density in the solder dipping bath. As a result, an Sn—Cu intermetallic compound having a high-melting point, not melting at a predetermined soldering temperature in the solder dipping bath develops. The compound adheres to the work to be soldered. Soldering quality is thus degraded. A density control technique has been proposed to overcome this problem in Japanese Open Gazette No. 2001-237536. According to the technique, additional solder having low Cu density is supplied to the solder dipping bath to control the Cu density to a constant density level or below.